The subject matter of the present invention relates to an induction logging sonde adapted to be disposed in a wellbore including a "folded" sensor array, the sensor array including a transmitter coil and a plurality of bucking coils and receiver coils, the plurality of bucking coils and receiver coils further including a plurality of cowound bucking coils and receiver coils, each of the plurality of cowound bucking coils and receiver coils being cowound together on the same bobbin, the transmitter coil being located at an end of the array, the plurality of bucking coils and receiver coils including the plurality of cowound bucking coils and receiver coils being located only on one side of the transmitter coil thereby shortening the sensor array of the present invention relative to the prior art sensor array of other prior art induction logging tools.
Over the past two decades, the electronics industry has been packing more and more circuitry into smaller and smaller packages. However, in the past decade and half, since digital well logging tools were introduced, the tendency has been to pack more circuitry into the logging tools. Examples of such circuitry packed into the logging tools include self-test, automatic calibration, more measurement channels, more sensors, etc. As a result, a conventional well logging toolstring combination of today is longer, in length, than its counterpart toolstring of fifteen years ago. The measurements, made by the conventional well logging toolstring, are more accurate and plentiful, and the environmental corrections are more automatic, but the increased toolstring length, of the conventional well logging tool, means that more rathole must be drilled and the rig-up time (the time it takes to assemble the toolstring) takes much longer.
One example of a conventional well logging induction tool is disclosed in U.S. Pat. No. 5,157,605 to Chandler et al, entitled "Induction Logging Method and Apparatus Including Means for Combining In-Phase and Quadrature Components of Signals Received at Varying Frequencies and Including use of Multiple Receiver Means Associated With a Single Transmitter", the disclosure of which is incorporated by reference into this specification.
In the Chandler patent, the induction tool includes an induction array, and the induction array includes a transmitter coil, a receiver coil disposed on both sides of the transmitter coil, and a secondary or bucking coil disposed on both sides of the transmitter coil and interposed between the transmitter coil and each receiver coil. While the measurements made by the induction logging tool of the Chandler patent are accurate and plentiful, the length of the induction logging tool string which included the induction array of the Chandler patent is approximately forty (40) feet. As a result, it may be difficult to place the induction logging toolstring of the Chandler patent in wellbores having a minimum rathole, or in wellbores having severe dog legs, or in horizontal wellbores having a short kickoff radius.
As a result, there is a need for a new induction logging tool, in accordance with the present invention, which has a toolstring length (16 feet) that is shorter than the toolstring length (40 feet) of the induction logging tool of the prior art Chandler patent. As a result, the new induction logging tool of the present invention is less prone to getting stuck in wellbores having poor borehole conditions, and it will provide logs in minimum rathole, it may be placed into wellbores having severe dog legs, and it may be placed into horizontal wellbores having a short kickoff radius.